JPH10291861A - Production of ceramic composite material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To profitably produce a high density ceramic composite material capable of producing an especially superior neutron controlling material by mixing boron carbide powder having a specified average particle diameter with hexagonal boron nitride powder having a specified average particle diameter in a specified ratio and hot-pressing the resultant mixture at a specified temp. SOLUTION: A mixture of 40-95 vol.% boron carbide powder having <=3 μm average particle diameter with 60-5 vol.% hexagonal baron nitride powder having >=15 μm average particle diameter is filled into dies of graphite, etc., and hot-pressed at <=2,100 deg.C. The hexagonal boron nitride particles during sintering have high orienting property in the pressing direction and produce anisotropy in mechanical characteristics and heat conductivity. When the resultant composite material having >=90% relative density is used as a neutron controlling material, high neutron absorbing ability is ensured, breaking due to swelling and thermal shock is suppressed, the damage of a cladding tube is considerably reduced and the useful life of a neutron controlling rod is prolonged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a high-density boron carbide / boron nitride ceramic composite material. The ceramic composite material produced by the present invention is used, for example, as a neutron control material used in a neutron irradiation environment.

[0002]

2. Description of the Related Art As a neutron controlling material, a boron compound having a large neutron absorption cross-sectional area has been conventionally used, and in particular, boron carbide having a high boron content per unit volume has been used. Boron has 10 and 11 mass isotopes, and boron having a mass of 10 has a high neutron absorption cross-section. Therefore, the concentration of naturally occurring boron having a mass of 10 is 19.8% to 9%.
Boron carbide concentrated to about 0% is widely used.
In particular, in a nuclear reactor utilizing high-speed neutrons, a boron carbide sintered body having a relative density of 90 to 95%, in which boron having a mass number of 10 is concentrated, is used as a neutron control material in order to enhance neutron absorption capacity. I have.

When a boron carbide sintered body is used as a neutron controlling material, it is generally used as a neutron control rod having a structure in which a cylindrical boron carbide sintered body is filled in a stainless steel cladding tube. The neutron absorption reaction of boron is (n,
α) reaction, swelling (swelling) occurs in the boron carbide sintered body due to the formation of helium accompanying the absorption of neutrons,
There is a problem that the mechanical interaction with the cladding tube is caused to shorten the life of the control rod.

[0004] Further, since the boron carbide sintered body generates a large amount of heat when absorbing neutrons, a large thermal shock is generated and the boron carbide sintered body is broken, which damages the cladding tube.
There is a problem that the life of the cladding tube is further shortened.

To solve these problems, for example, there are the following proposals. (1) A method of suppressing crystal grain growth in the sintering process using a boron carbide raw material having a specified B / C atomic ratio and primary particle size (Japanese Patent Publication No. 4-78159). (2) A method of providing a buffer portion made of solid boron nitride in a cladding tube (Japanese Patent Laid-Open No. 59-150369). (3) A method of adding 30% by volume or less of boron nitride to boron carbide to improve the thermal shock resistance of the boron carbide sintered body (Proc.
11th Int. Symp. Boron, Borides and RelatedCompound
s, Tsukuba, 1993 JJap Series 10 (1994) pp216-21
9).

[0006]

However, (1)
Although the swelling property is reduced by the technique of the above, the danger that the boron carbide sintered body breaks due to thermal shock and damages the cladding tube remains unchanged. According to the technique (2), even if swelling or breakage of the boron carbide sintered body due to thermal shock occurs, a neutron control rod that does not easily damage the cladding tube can be obtained, but the structure of the neutron control rod becomes complicated. is there. According to the technique (3), the thermal shock resistance of the neutron control material (boron carbide / boron nitride composite material) is improved, and the control material can be prevented from being broken by the thermal shock and resulting damage to the cladding tube. , Boron carbide /
When the hot pressing temperature of the boron nitride composite material is about 2100 ° C., the relative density is only about 80%, and the neutron absorption capacity is low, which is not practical. In addition, the hot press temperature is 2100
When the temperature exceeds ℃, the graphite die generally used as a die reacts with boron carbide and boron nitride, and the load on the graphite die becomes large, and the die is liable to be broken.

Therefore, there is a demand for an economical neutron control material which is less likely to cause damage to the cladding tube due to swelling and thermal shock of the neutron control material during neutron absorption and which has excellent neutron absorption efficiency.

[0008] An object of the present invention is to have a high neutron absorption capacity,
An object of the present invention is to provide a method for economically producing a high-density boron carbide / boron nitride-based ceramic composite material capable of producing a neutron control material that is less likely to be broken by swelling or thermal shock during neutron absorption.

[0009]

That is, the present invention relates to a method for producing a powder of boron carbide having an average particle size of 3 μm or less at 40 to 95% by volume and a hexagonal boron nitride powder having an average particle size of 15 μm or more at 60 to 5% by volume. % At 2100 ° C.
A method for producing a ceramic composite material having a relative density of 90% or more, characterized by hot pressing at the following temperature.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

The boron carbide powder used in the present invention has an average particle size of 3 μm or less as measured by the microtrack method, and particularly preferably has an average particle size of 2.5 μm or less. If the average particle size exceeds 3 μm, it becomes difficult to uniformly disperse the boron carbide particles at the grain boundaries of the hexagonal boron nitride particles, and the density decreases. The smaller the average particle size is, the more preferable.
m, the oxygen content in the boron carbide powder is 3% by weight.
Therefore, the oxygen content in the sintered body increases, and the sintered body becomes unsuitable for use as a neutron control material.

The hexagonal boron nitride powder used in the present invention has an average particle size of 15 μm or more, as measured by the microtrack method, and particularly preferably has an average particle size of 20 μm or more. When the average particle size is less than 15 μm, the density of the sintered body is reduced, or the oxygen content in the hexagonal boron nitride powder easily exceeds 1% by weight, which is not suitable for use as a neutron controlling material. The larger the average particle diameter is, the more preferable. However, generally available particles have an average particle diameter of about 40 μm.

Further, the relative density in the present invention is a percentage of an actually measured density value with respect to the theoretical density of a composite material calculated from the respective blending amounts using the theoretical densities of boron carbide and hexagonal boron nitride. For example, the theoretical density of boron carbide consisting of boron Naturally occurring 2.52 g / cm ^3, hexagonal boron nitride is calculated as 2.26 g / cm ^3.

The mixing ratio of the boron carbide powder and the boron nitride powder used in the present invention is 40 to 95% by volume of the boron carbide powder,
Boron nitride powder is 60 to 5% by volume. 4 boron carbide powder
If the content is less than 0% by volume or the content of boron nitride powder exceeds 60% by volume, the relative density decreases. On the other hand, if the content of boron carbide powder exceeds 95% by volume or the content of boron nitride powder is less than 5% by volume, the thermal shock resistance of the sintered body decreases, and the sintered body becomes unsuitable for use as a neutron control material.

When the ceramic composite material of the present invention is used as a neutron controlling material, it is generally used after being filled in a stainless steel cladding tube and at a high temperature, so that it does not cause a chemical reaction with the stainless cladding tube. It is preferable that the oxygen content of the ceramic composite material of the present invention is as small as possible.

The mixing of the boron carbide powder and the boron nitride powder may be performed using a general ball mill, vibration mill, ribbon blender, Henschel mixer, or the like, and there is no particular limitation as long as they can be uniformly mixed.

In the present invention, the mixed raw material powder is filled in a die such as a graphite material and hot-pressed at a temperature of 2100 ° C. or less. If the hot pressing temperature exceeds 2100 ° C., the graphite material of the die reacts with boron carbide or boron nitride to adversely affect the sintered body or increase the load on the die and shorten its life. On the other hand, if the hot pressing temperature is lower than 1900 ° C., the sintered body density tends to be low, which is not preferable.

Since the ceramic composite material of the present invention is produced by hot pressing a mixed raw material powder containing a hexagonal boron nitride powder having a large average particle diameter as one component, the hexagonal boron nitride particles in the sintered body are produced. Has a stronger orientation in the pressing direction than before, and anisotropy occurs in mechanical properties and thermal conductivity. As a result, it has swelling resistance and thermal shock resistance expected as a neutron control material.

When the ceramic composite material of the present invention having a relative density of 90% or more, particularly 95% or more, is used as a neutron control material among the composite materials produced by the above method, the neutron absorption capacity is high, and swelling and thermal shock And the damage to the cladding tube can be greatly reduced, the life of the neutron control rod can be prolonged, and the safety and economy can be improved.

[0020]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

Examples 1-3 Comparative Examples 1-4 Boron carbide and hexagonal boron nitride powder consisting of naturally occurring boron shown in Table 1 were mixed in a vibration mill at the compounding ratio shown in Table 2. This mixed raw material powder is filled in a graphite die,
Hot press molding at the temperature shown in Table 2, diameter 50mm,
A ceramic composite material having a height of 50 mm was manufactured. The press pressure was 25 MPa.

With respect to the obtained ceramic composite material,
The dimensions and weight were measured, the density was calculated, and the relative density was determined. In Comparative Example 4, after hot pressing, it partially reacted with the graphite die, and could not be taken out of the die.

From the ceramic composite material manufactured in Example 2, test specimens of 3 × 4 × 40 mm were processed and sampled from the direction parallel and perpendicular to the hot pressing direction, respectively.
When the four-point bending strength at room temperature was measured by a method based on JISR1601, the bending strength in the parallel direction and the vertical direction was 102 MPa and 33 MPa, respectively. A similar test piece was heat-treated in a furnace at 800 ° C. for 10 minutes, dropped into water at room temperature and quenched, and then similarly measured for four-point bending strength, but no decrease in bending strength was observed. Was.

Further, test pieces each having a diameter of 10 mm and a thickness of 1.5 mm were processed and sampled from the ceramic composite material produced in Example 2 in a direction parallel to and perpendicular to the hot pressing direction, and were processed in accordance with JISR1611. When the thermal conductivity at room temperature was measured, the thermal conductivity in the parallel direction and the thermal conductivity in the vertical direction were 49 W / mK and 20 W / mK, respectively.
Met.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

The ceramic composite material produced according to the present invention has high density and strong anisotropy.
The neutron control material manufactured by using it, the mechanical interaction with the cladding tube due to swelling and thermal shock can be greatly reduced, and the safety and economy of the neutron control rod can be improved compared to the conventional material. Can be achieved.

Claims (1)

[Claims] 1. A mixed raw material powder containing 40 to 95% by volume of boron carbide powder having an average particle size of 3 μm or less and 60 to 5% by volume of hexagonal boron nitride powder having an average particle size of 15 μm or more, A method for producing a ceramic composite material having a relative density of 90% or more, characterized by hot pressing at a temperature of not more than ℃.